Yes, the product description states explicitly that the spray is also intended for treating the inner surface of footwear. This makes sense: closed shoes are a warm, humid environment in which unwanted microflora multiplies actively. Treating shoes between wears helps to maintain hygiene in places where the product does not come into direct contact with the skin.
In practice, a few sprays into each shoe or trainer are enough, especially into the toe area and over the insole. After applying, let the shoes air a little before wearing them again. The spray format is convenient because the product is distributed in a thin layer and leaves no visible marks on the fabric or leather of the insole.
For the skin of the feet and the nails, the method is the same: apply to a clean, dry surface, paying attention to the spaces between the toes and the nail plates, places that a cream or gel has more difficulty reaching. The spray format is particularly practical here precisely because of these hard-to-reach areas.